The invention relates to a system and a method for controlling an energy storage device, in particular in a battery direct inverter circuit in order to generate an n-phase alternating current voltage.
It is becoming apparent that in the future both in the case of stationary applications, such as for example wind turbines or solar panels, and also in vehicles, such as hybrid or electric vehicles, electronic systems will increasingly be used, which electronic systems combine new energy storage technologies with electrical drive technology.
The supply of multiphase current into an electrical machine is generally provided by means of a converter in the form of a pulse width modulated inverter. For this purpose, a direct current voltage that is provided by a direct current voltage intermediate circuit can be converted into a multiphase alternating current voltage, by way of example a three-phase alternating current voltage. The direct current voltage intermediate circuit is supplied by a string of series-connected battery modules. Several battery modules are frequently connected in series in a traction battery in order to be able to fulfill the relevant requirements relating to performance and energy for a particular application.
A battery system having an integrated inverter function is described in the publication U.S. Pat. No. 5,642,275 A1. Systems of this type are known by the name multilevel cascaded inverter or also battery direct inverter (BDI). Systems of this type comprise direct current sources in several energy storage module strings, which can be connected directly to an electrical machine or an electrical network. Single-phase or multiphase supply voltages can be generated. The energy storage module strings comprise several series-connected energy storage modules, wherein each energy storage module comprises at least one battery cell and an allocated controllable coupling unit that renders it possible in dependence upon control signals to bridge the respective allocated at least one battery cell or to connect the respective allocated at least one battery cell into the respective energy storage module string. Optionally, the coupling unit can be designed so as to render it possible in addition to connect the respective allocated at least one battery cell also having reverse polarity into the respective energy storage module string, or also to disconnect the respective energy storage module string. By virtue of the fact that the coupling units are suitably controlled, for example with the aid of pulse width modulation, it is also possible to provide suitable phase signals for controlling the phase output voltage so that a separate pulse width modulated inverter can be omitted. The pulse width modulated inverter that is required in order to control the phase output voltage is consequently, in a manner of speaking, integrated into the BDI.
BDIs generally comprise a higher efficiency level and are more reliable in comparison to conventional systems. The reliability is ensured, amongst other things, by virtue of the fact that defective battery cells that have failed or are not fully functional can be disconnected from the energy supply strings by virtue of suitably controlling the bridging of the coupling units. The phase output voltage of an energy storage module string can be varied by correspondingly controlling the coupling units and it can in particular be adjusted in steps. The stepped adjustment of the output voltage is determined from the voltage of an individual energy storage module, wherein the maximum possible phase output voltage is determined by the total of the voltages of all of the energy storage modules of one energy storage module string.
Coupling units can be controlled in a pulse width modulated manner (PWM) in order to adjust an output voltage of an energy storage module. As a consequence, it is possible by purposefully varying the switch-on and/or switch-off times to provide a desired mean value as an energy storage module voltage.
A method for controlling several series-connected step-down converters in a pulse width modulated manner is disclosed in the publication DE 39 24 398 A1, in which method all the step-down converters that are series-connected on the output side are operated with the aid of a uniform phase control factor and a uniform pulse width modulated period. The output voltage progressions of the individual step-down converters are in each case offset with respect to one another by a fraction of a control period. As a consequence, it is possible to maintain a total output voltage that comprises a smaller range of fluctuation from its mean value.
With respect to BDIs, there is a need for pulse width modulated controlling methods in which the fluctuation of the total output voltage can be optimized, in particular whilst generating an n-phase output voltage system in an n-phase BDI.